1. Field of the Invention
The present invention relates to a multilayer printed wiring board and method for manufacturing the same, and more particularly to a multilayer printed wiring board having an interstitial via hole (IVH) structure and a method for manufacturing the same.
2. Description of the Related Art
A conventional multilayer printed circuit board is constituted by a laminate obtained by alternately laminating copper-clad laminates and prepregs. The laminate has a surface on which a surface wiring pattern has been formed. Moreover, an internal wiring pattern is formed between interlayer insulating layers. Through holes are, by punching formed in the direction of the thickness of the laminate to establish the electric connections among internal wiring patterns or among internal wiring patterns and the surface wiring patterns.
The multilayer printed circuit board having the above-mentioned through hole structure must have regions for forming the through holes. Therefore, the density at which elements are mounted cannot be easily raised. As a result, there arises a problem in that the multilayer printed circuit board having the through hole structure cannot easily address needs for considerable reductions in the sizes of the portable electronic apparatuses, realizing narrow pitch packages and practical use of MCM.
As an alternative to the foregoing multilayer printed circuit board having the through hole structure, a multilayer printed circuit board has recently received attention which is formed into a full-thickness interstitial via-hole (IVH) structure which is able to easily address raising of the density.
The multilayer printed circuit board having the full-thickness IVH structure is a printed circuit board having a structure in which via holes for electrically connecting conductive layers to one another are formed in each of the interlayer insulating layers which constitute the laminate. That is, the foregoing printed circuit board has the via holes (buried via holes or blind via holes) which do not penetrate the substrate on which the circuit is formed and which electrically connect the internal wiring patterns to one another or the internal wiring patterns and the surface wiring patterns to one another. Therefore, the multilayer printed circuit board having the IVH structure is free from a necessity of specially forming regions for forming the through holes. Therefore, arbitrary layers can freely be connected to one another through small via holes. As a result, size reduction, high density and high-speed propagation of signals can be easily realized.
The multilayer printed circuit board having the IVH structure is manufactured by a process arranged, for example, as shown in FIG. 6.
Initially, a material having a structure in which an aramide nonwoven fabric cloth is impregnated with epoxy resin is employed as prepreg 112. Then, an operation for forming holes in the prepreg 112 is performed by using carbon dioxide gas laser. Then, conductive paste 114 is enclosed in obtained hole portions 112a (see FIG. 6 (A)).
Then, copper foil 116 is laminated on each of the two sides of the prepreg 112, and then heat and pressure are applied to the prepreg 112 having the copper foil 116 by heat pressing. Hence it follows that the epoxy resin and the conductive paste of the prepreg 112 are hardened so that the electrical connection between the two copper foil members 116 on the two sides of the prepreg 112 is established (see FIG. 6 (B)).
Then, the copper foil 116 on each side is patterned by an etching method so that a hard and double-sided substrate having via holes is obtained (see FIG. 6 (C)).
Then, the obtained double-sided substrates are used as core layers to form a multilayer structure. Specifically, the prepreg and copper foil are sequentially laminated on the two sides of the foregoing core layer while the prepreg and the copper foil are being aligned. Then, heat pressing is again performed, and then the uppermost copper foil 116 is etched. Thus, a four-layer substrate is obtained (see FIGS. 6 (D) and 6 (E)). When a structure having a large number of layers is formed, the foregoing process is repeated. Thus, a six-layer substrate or an eight-layer substrate can be obtained.
The foregoing conventional technique, however, must repeat the laminating process using heat pressing and the process for patterning the copper foil by performing the etching operation. Therefore, the manufacturing process becomes too complicated and considerable time is required to complete the manufacturing operation.
If the multilayer printed circuit board having the IVH structure which can be obtained by the above-mentioned manufacturing method encounters only one defective portion (only one defective process) in the patterning operation during the manufacturing process, the overall circuit board, which is the final product, becomes a defective product. Hence it follows that the manufacturing yield deteriorates excessively.
To overcome the above-mentioned problems, an object of the present invention is to provide a high-density multilayer printed circuit board having the IVH structure which can be manufactured by a very simple process and which permits a satisfactory high manufacturing yield to be realized and a manufacturing method therefor.
To achieve the above-mentioned object, the present invention is structured as follows.
According to one aspect of the present invention, there is provided a method of manufacturing a multilayer printed circuit board comprising the steps (1) to (5):
(1) a step for forming a non-penetrating hole in an insulating base member, such as an organic insulating base member, having a metal layer formed on either surface thereof by a laser irradiation operation such that the non-penetrating hole reaches the metal layer;
(2) a step for forming via holes by enclosing a conductive material into the non-penetrating hole formed in step (1);
(3) a step for forming a conductive circuit by etching the metal layer;
(4) a step for forming projecting conductors on the surfaces of the via holes to form a single-sided circuit substrate;
(5) a step for laminating the single-sided circuit substrate and another single-sided circuit substrate or the single-sided circuit substrate and another substrate obtained in the steps (1) to (4) such that the projecting conductors of the single-sided circuit substrate and a conductive circuit of the other circuit are positioned opposite to each other through non-hardened resin which is an organic adhesive layer and applying heat and pressure to (heat-pressing) the laminate.
According to another aspect of the present invention, there is provided a method of manufacturing a multilayer printed circuit board comprising the steps (1) to (4):
(1) a step for forming a non-penetrating hole in an insulating base member, such as an organic insulating base member having a metal layer formed on either surface thereof by a laser irradiation operation such that the non-penetrating hole reaches the metal layer;
(2) a step for forming via holes by enclosing a conductive material into the non-penetrating hole formed in step (1) and forming projecting conductors on the surfaces of the via holes;
(3) a step for forming a conductive circuit by etching the metal layer;
(4) a step for laminating the single-sided circuit substrate and another single-sided circuit substrate or the single-sided circuit substrate and another substrate such that the projecting conductors of the single-sided circuit substrate and a conductive circuit of the other single-sided circuit substrate or the other conductive circuit are positioned opposite to each other through non-hardened resin which is an organic adhesive layer and applying heat and pressure to (heat pressing) the laminate.
In the foregoing aspects of the present invention, the projecting conductors are inserted into the non-hardened resin so that resin is squeezed out. That is, the projecting conductors penetrate the organic adhesive layer so as to be electrically connected to another substrate.
According to another aspect of the present invention, there is provided a multilayer printed circuit board obtainable from the above-mentioned manufacturing process.
That is, a multilayer printed circuit board having a structure that single-sided circuit substrates each of which has a conductive circuit formed on either side of an organic insulating base member thereof and via holes formed in the organic insulating base member by enclosing a conductive material are connected to other substrates each having a conductive circuit through organic adhesive layers, the multilayer printed circuit board comprising: projecting conductors formed on the surfaces of the via holes opposite to the surface of the organic insulating base member on which the conductive circuit is formed, wherein the projecting conductors are inserted to penetrate the adhesive layers so as to be connected to the conductive circuits of the other substrates.
The multilayer printed circuit board and a manufacturing method therefor according to the present invention are arranged such that the single-sided circuit substrates each having a conductive circuit which incorporates a predetermined wiring pattern formed thereon are previously and individually manufactured. Therefore, inspection for detecting whether or not the conductive circuit or the like has a defective portion can be performed before the single-sided circuit substrates are laminated. Hence it follows that only single-sided circuit substrates free from any defect can be used in the laminating process. That is, the manufacturing method according to the present invention is able to reduce defects in the manufacturing process. As a result, the multilayer printed circuit board having the IVH structure can be manufactured with a high manufacturing yield.
The method of manufacturing a multilayer printed circuit board according to the present invention is not required to repeat the heating press operation while the prepreg is being laminated as distinct from the conventional technique. That is, the present invention enables the heat pressing operation to be completed at a time such that a plurality of the single-sided circuit substrates are laminated through the adhesive agents placed on the single-sided circuit substrates. Therefore, the manufacturing method according to the present invention is free from a necessity for repeating the laminating process, in which the complicated heat press is performed, and the patterning process. Hence it follows that the multilayer printed circuit board having the IVH structure can be efficiently manufactured.
In the present invention, formation of the non-penetrating holes in the organic insulating substrate is performed by laser machining. The structure of the present invention is able to eliminate the necessity of forming holes in the organic adhesive agent by performing a laser process. That is, after holes have been formed in the organic insulating substrate by performing the laser process, the organic adhesive layer can be formed on the single-sided circuit substrate or the substrate having the conductor circuit.
That is, the present invention is structured such that the projecting conductors inserted into the organic adhesive layer during the heat pressing establish the connection of the conductor circuits. Therefore, previous formation of the conducting hole in the organic adhesive layer is not required. The organic adhesive layer may be formed at the final heat pressing process. As a result, the desmear process, which is performed after the hole has been formed in the process for manufacturing the single-sided circuit substrate, may be performed before the formation of the organic adhesive layer. Hence it follows that the desmear process does not erode the organic adhesive layer.
Also in a case where the non-penetrating hole is filled with the electrolytic plating, the organic adhesive layer can be formed on the single-sided circuit substrate or the substrate having the conductive circuit after the non-penetrating hole has been formed in the organic insulating base member by performing the laser process and the non-penetrating hole has been filled with the electrolytic plating. Therefore, the electrolytic plating solution and the organic adhesive layer are not made contact with each other. As a result, the erosion and contamination of the organic adhesive layer with the plating solution can be prevented.
Since the organic adhesive layer is not hardened until the heat pressing process which is the final process is performed, the organic adhesive layer easily deteriorates owing to the desmear process and the plating solution. The present invention is characterized in that the foregoing problem can be prevented and a reliable substrate can easily be formed.
Moreover, a necessity for previously forming a conducting hole in the adhesive layer can be eliminated. Therefore, defective conduction caused from deviation in the positions of the hole in the adhesive layer and the projecting conductors provided for the organic insulating base member can be prevented.
In the present invention, the projecting conductors are formed on the via holes filled with the conductive paste or the electrolytic plating. Therefore, electrical connection between the upper and lower conductive layers can be easily established by penetrating the relatively thin organic adhesive layer. Therefore, the height and the diameter of each projecting conductor can be reduced. Hence it follows that the pitch between adjacent projecting conductors can be shortened. Therefore, the pitch between adjacent via holes can also be shortened. As a result, addressing to raising of the density can be permitted.
In a case where the via holes are filled with the electrolytic plating, the resistance value between the upper and the lower conductor layers can be lowered.
Techniques for connecting the upper and lower conductors to each other by penetrating the resin insulating layer have been disclosed in Japanese Patent Laid-Open No. 7-14628, Japanese Patent Laid-Open No. 7-106756, Japanese Patent Laid-Open No. 7-231167, Japanese Patent Laid-Open No. 8-172270 and Japanese Patent Laid-Open No. 8-288649. The foregoing techniques are different from the technique with which the projecting conductors formed on the via holes filled with the foregoing material are caused to penetrate only the organic adhesive layer to connect the upper and lower conductors to each other. Therefore, the effect of the present invention cannot be obtained.